Several eukaryotic DNA viruses maintain their genomes as extrachromosomal multicopy nuclear episomes in proliferating host cells. Such episomal maintenance is characteristic of latent infection of the Bovine papillomavirus type 1 (BPV-1), Epstein-Barr Virus (EBV) as well as for Kaposi sarcoma associated Human herpesvirus type 8 (HHV8). The latency of the viral genome in dividing cell population requires activity of the viral genome at the two phases of the cell cycle: the viral genome replication during the S phase and proper segregation and partitioning of the replicated genomes into daughter cells during the host cell mitosis. For BPV-1 and two members of the gammaherpesvirus family—EBV and HHV8 an effective segregation of viral genomes into daughter cells and nuclear retention during mitosis is mediated through a single viral protein serving as a molecular linker, which attaches viral genomes to the host mitotic chromosomes. This linker protein is a viral regulatory protein E2 for BPV-1, viral transactivator EBNA1 for EBV and viral transcription repressor LANA1 for HHV8.
For initiation of the BPV-1 DNA replication in vivo, minimal origin region in cis and two viral proteins—E1 and E2, in trans, are absolutely essential. However, the minimal origin (MO) is not sufficient for stable extrachromosomal replication in dividing cells. An additional element, the Minichromosome Maintenance Element (MME) ensures the long-term episomal persistence of the genome in the presence of viral E1 and the E2 proteins in the dividing cells. In the BPV-1 genome in total 17 E2 protein binding sites with different affinity to E2 can be identified: 12 of these are locating in the noncoding upstream regulatory region (URR). We have shown that the minichromosome maintenance element (MME) activity can be provided by the subset of the E2 binding sites. The function of multimeric E2 protein binding sites in the stable maintenance of the BPV-1 genomes is to provide the anchoring function for the E2 protein, which therefore tethers MME containing plasmids to mitotic chromosomes. This linkage between the BPV-1 genome and host chromatin ensures also that the viral genome is targeted to the nucleus when the nuclear membrane is reassembled during mitosis. In the case of EBV, the stable maintenance of replicated genomes is achieved due to the EBNA1 protein and FR-element, which is comprised of multimeric EBNA1 protein binding sites.
We have shown that the BPV1 E2 protein dependent MME (Abroi et al. 2004) and EBV EBNA1 dependent FR (Mannik, Janikson and Ustav, unpublished) segregation/partitioning activities function independently from replication of the plasmids. The stable-maintenance funcion of EBNA1/FR has been used to ensure long-time episomal maintenance for cellular replication origins. The E2/MME-dependent stable-maintenance function has never been tested with heterologous replication origins.
Transfection or infection of permissive cells with polyomavirus genome or replicator results in amplificational replication leading to cell death due to the over-replication. The mechanism of the BPV-1 origin based episomal replication is more complex and controlled. On one hand the first amplificational replication step, resembling in many aspects polyomavirus lytic over-replication is crucial for establishment of the stable episomal replication of the papillomavirus DNA. Such replication leads to increase of expression level of the viral proteins and copy-number of the viral genome. Increase of the E1 protein concentration, however, over certain limit induces the “onion-skin” type replication of the BPV-1 origin and generation of the replication intermediates having tendency for high frequency of DNA rearrangements and integration of the fragments of the viral DNA into chromosomal DNA. To maintain the stability and intactness of the viral genome, virus has to apply certain mechanism to assure proper balance between initiation and elongation of replication fork as well as segregation/partitioning of the viral plasmids during cell division
The stable episomal maintenance systems described earlier (U.S. Pat. No. 6,479,279) were provided with homologous replication origins. Characteristic for these systems is for example a high mutation frequency, especially recombination. Furthermore, the system does not give stable replication in single cells but part of the cells lose their plasmid in every generations. This fact creates serious limits for the system to be used for example in protein production. Nilsson et al (1991) describe a system providing an enhancer function, but that system has serious limitation in terms of stability. The present disclosure provides improvements over the problems encountering prior systems. The present disclosure provides an extended episomal maintenance system with heterologous replication origin. The heterologous system according to this disclosure can be used in combination with homologous systems. The present disclosure provides a system where plasmids containing core polyoma virus origin (not containing enhancer function) partition into daughter cells. The system according to this disclosure provides a system where a unique configuration of polyoma virus origin (core polyomavirus origin not containing enhancer function) is stable and is segregated in the presence of helper proteins. An advantage of the present system is that episomal state of chimeric origins is maintained without rearrangement. The present invention further provides a possibility to combine vectors with polyoma virus origin and papilloma origin into a single cell, thereby enabling expression of more than one different recombinant proteins or RNAs in one cell. A further advantage of the current system is that it provides stable episomal maintenance in the cell that lasts up to several months as opposed to all previous systems which provide maintenance of maximally a few weeks; e.g. U.S. patent application Ser. No. 10/938,864 describes a system capable of stable episomal replication lasting about two weeks.
The problem that we aimed to solve is that stable expression of gene products in cell systems is costly and time consuming. This problem has been solved in the present disclosure by transient expression of gene products in cells. Since the genes of interest are replicated and maintained outside the chromatin in the nucleus, the vectors go to the progeny in cell division and segregation and with the method of the current disclosure the expression of the gene products can be continued for months.
Development of stable expression in cell lines takes usually several months or even years. The transient system that we describe here is much faster and therefore useful and novel. On the other hand the transient systems so far known have a very limited half-life; i.e. maximally a couple of days. In addition, those systems may need construction of recombinant viruses which makes the systems expensive and very time consuming. Our system provides a marked improvement to the existing art; the system according to this disclosure provides a transient expression system that maintains the expression levels for several weeks and even up to several months.
The present disclosure provides a possibility to develop stable cell lines when the vector according to this disclosure contains a selection marker and the cells are cultivated on a medium containing the selective agent. The present disclosure also provides a possibility to express gene products in a cell line for shorter time when the vector does not contain a selection marker. However, even without using selection pressure the current system provides stable maintenance that lasts longer than with any other comparable system previously known.
The present disclosure further enables development of a multi-replicon expression system, where more than one gene products are expressed from different replicons and the replicons are locating in same cell. Such a mechanism is useful for example to express different subunits of antibodies or enzyme subunits in one cell or to study interactions of macromolecules expressed in the cell.
An object of the present disclosure is to provide a mechanism to extended episomal maintenance of polyoma virus core origin.
Another object of the present disclosure is to provide a mechanism to extended episomal maintenance of polyoma virus core origin without selective pressure.
Another object of the present disclosure is to provide constructs in conjunction with the segregations/partitioning elements from BPV1 or EBV.
A still further object of the present disclosure is to provide cell lines capable of supporting the replication and episomal maintenance of hybrid plasmids.
A still further object of the present disclosure is to provide a transient system for long lasting expression of gene products in eukaryotic cells.
An even further object of the present disclosure is to provide cell lines harboring more than one different vectors and thereby providing expression of more than one different genes of interest.
An even further object of the present disclosure is to provide a transient system for long lasting production therapeutic, prophylactic or endotoxine free gene products for diagnostic and other applications in eukaryotic cells.
Another object of the present disclosure is to provide a transient system for long lasting production of RNA or proteins in eukaryotic cells. The cells can be cultivated and gene products can be expressed in small and large scale; from laboratory flasks and Petri dishes up to big fermenters.
In order to study coordination between initiation and elongation of replication and segregation/partitioning of the episomal origin plasmids, we designed several hybrid replication origins comprising polyomavirus replication origin and minichromosome maintenance element (MME) of the BPV1. We analyzed the effect of the PyV enhancer and E2 dependent enhancer on the functionality of the PyV core origin in establishing of the extended episomal maintenance replication of the hybrid origins in the cell lines expressing E2 proteins and PyV large T antigen (LT). Additionally, we studied the functions of the BPV1 E2 protein necessary for maintenance function of the hybrid origins and found that transcriptional activation function of the E2 protein is unable to promote the establishment of the stable replication. Similar hybrid origins comprising the EBV FR-element and polyomavirus replication origin were constructed and studied in the cell lines expressing EBNA1 and polyomavirus large T antigen (LT). Our data suggest convincingly that segregation/partitioning functions of the BPV-1 and EBV can effectively be used for extended episomal maintenance of the polyomavirus core origin.
Bovine papillomavirus type 1, Epstein-Barr virus and Human Herpesvirus type 8 genomes are stably maintained as episomes in dividing host cells during latent infection. Segregation/partitioning function is given to these origins by single viral specific DNA-binding protein and multimeric protein binding sites. This disclosure shows that the BPV-1 E2 protein-dependent MME comprising E2 multimeric binding sites can provide the extended maintenance replication function to the mouse polyomavirus (PyV) core origin plasmids in the presence of BPV-1 E2 protein and PyV large T-antigen (LT), but fail to do so for the complete PyV origin. In mouse fibroblast cell-lines expressing PyV LT and BPV-1 E2 (COP5/E2), the plasmids carrying PyV core origin linked to at least five multimeric E2 protein binding sites show the capacity for long term episomal replication, which can be monitored for more than 5 months (under selective conditions). Overall structural integrity as well as the intactness of domains of BPV-1 E2 are required for efficient episomal maintenance. Our data show clearly that the large T antigen dependent replication function of the polyomavirus and extended maintenance functions of the BPV-1 are compatible in certain configurations. Further quantitative analysis of the loss of the episomal plasmids carrying hybrid origin showed that MME dependent plasmids are lost with the frequency of 6% per generation. We also constructed the plasmids where PyV core origin maintenance function/replication activation could be provided by Epstein-Barr virus Family of repeats (FR-element) and EBNA1 protein. The maintenance of the Polyomavirus core origin plasmid was characterized by 13% loss of the plasmid during one cell generation in the case of these plasmids. Our data clearly indicate that maintenance functions from different viruses are interchangeable and can provide segregation/partitioning function to different heterologous origins in variety of cells and be used in expression of gene products.